Extracting power from a fluid flow

ABSTRACT

An apparatus for extracting power from a fluid flow comprises a fluid driveable engine, a conduit, disposed to enable fluid communication between a portion of the fluid flow, the fluid driveable engine and a transmission fluid, the fluid in the fluid flow and the transmission fluid being different fluids and the portion of the fluid flow being at a lower pressure than the transmission fluid by virtue of its flow rate, thus causing the transmission fluid to be drawn through the conduit to become entrained in the fluid flow, the fluid driveable engine being arranged such that the flow of the transmission fluid along the conduit acts to drive the fluid driveable engine.

This application is the U.S. national phase of international applicationPCT/GB03/01173, filed 19 Mar. 2003, which designated the U.S. and claimspriority to GB Application No. 0206623.1 filed 20 Mar. 2002. The entirecontents of these applications are incorporated herein by reference.

This invention relates to apparatus for extracting power from a fluidflow, such as a tidal stream.

With increasing public awareness of environmental pollution and inparticular, global warming there is a growing interest in renewableenergy sources. A 1994 survey of the energy available in sea or rivercurrents and tidal streams around the UK by the Department of Trade andIndustry's renewable energy unit at Harwell found that a considerablefraction of the UK's energy needs could be met if this energy could beharnessed.

The energy in the currents is kinetic rather than potential, which meansthat it has to be extracted in a different way from that employed in aconventional hydroelectric scheme. Typically, in a tidal streaminstallation, a turbine might be placed underwater in the tidal streamto extract the energy—an underwater equivalent of a wind powergenerator. For example, in a development funded by the European Union(then the European Community), it was planned to set up submarinepropeller-driven turbines in selected locations where the current flowsrapidly.

A disadvantage of these conventional underwater systems is that in orderto access the energy of the fluid flow the moving parts are placedunderwater in a hostile environment, making them prone to damage andinconvenient and costly to access and repair. Furthermore, if the wateris slowed too much (i.e. too big a fraction of the kinetic energy isextracted), then the head needed to drive it will be increased. Tominimise the required head, thereby obviating the need for a barrage,any turbine placed in the stream will have to have its blades highlyfeathered, making it uneconomic.

A solution to these deficiencies was provided by WO 99/66200, whichdiscloses a device for extracting energy from a fluid flow whereby thefluid is pumped away from the flow so that it can be led to a fluiddriveable engine, such as a turbine, sited at a position remote from theunderwater fluid flow. This solution avoided the requirement for movingparts underwater and the correspondingly high maintenance costs.

The above solution however is a relatively low pressure device, in whichthe water driving the fluid driveable engine flows fairly slowly (oforder 5 m/s) in comparison with what is commonplace in a typicalhydroelectric installation, This speed cannot be significantly increasedby constricting the pipe diameter without introducing punitive powerlosses, and so only a low speed water turbine can be driven by thisdevice. Such a turbine is not well matched to the requirements of anelectrical generator, which may run at typically 1500 rpm. In order tosupply a useful electrical output, a large and expensive gearbox wouldbe needed.

WO 99/66200 discloses an embodiment of the above invention in which analternative fluid to that present in the fluid flow is used to, drivethe fluid driveable engine, providing a solution to these problems, butonly at the expense of increasing the complexity of the apparatus andnecessitating the inclusion of sealed containers underwater near to thefluid flow.

U.S. Pat. No. 5,377,485 discloses an electric power conversion system inwhich air is used to drive a turbine prior to being entrained in a fluidflow. In some embodiments a complicated switching arrangement is used toseparate the air into packets within the fluid flow and avoid bubbleformation.

Various respective aspects and features of the invention are defined inthe appended claims.

According to one aspect of the present invention there is provided anapparatus for extracting power from a fluid flow, the apparatuscomprising: a fluid driveable engine, a conduit, disposed to enablefluid communication between a portion of the fluid flow, the fluiddriveable engine and a transmission fluid, the fluid in the fluid flowand the transmission fluid being different fluids the transmission fluidbeing a gas and the fluid flow being a liquid and the portion of thefluid flow being at a lower pressure than the transmission fluid byvirtue of its flow rate, thus causing the transmission fluid to be drawnthrough the conduit, the transmission fluid exiting the conduit via aplurality of entrainment outlet to become entrained in the fluid flow,the fluid driveable engine being arranged such that the flow of thetransmission fluid along the conduit acts to drive the fluid driveableengine, and the size of each of the plurality of entrainment outletsbeing that of a practical bubble size.

The apparatus of the present invention alleviates the disadvantages ofthe prior art by enabling an alternative fluid to that present in thefluid flow to drive the fluid driveable engine without the need foradditional active components such as valves and large underwaterstructures such as containers The use of an alternative fluid as thedrive fluid enables appropriate fluids to be chosen to provideadvantages such as a reduction in frictional losses and/or reduction incorrosion or erosion suffered by the device.

In an embodiment of the invention, the transmission fluid comprises air.An advantage of this is that the frictional losses involved intransmitting air under pressure over a large distance are very much lessthan those involved in transmitting water. Another advantage of usingair as the transmission fluid is that air can be entrained in (forexample) seawater without causing a pollution problem. Additionally, anincrease in flow rate could be achieved by forcing the transmissionfluid to flow through a narrow constriction without significantturbulent losses, where it could be used to drive a high-speed turbine,obviating the need for a gearbox. Furthermore, if the system is drivinga gas turbine, this arrangement acts to produce a reduced exhaustpressure for the gas turbine, which increases its efficiency. It shouldbe noted that this system is highly compatible with a gas turbinegenerator, in which hydrocarbons are used to supplement, say tidalenergy.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates a venturi apparatus in which air isentrained into a fluid flow;

FIG. 2 schematically illustrates an apparatus for extracting power froma fluid flow;

FIG. 3 schematically illustrates an embodiment of the invention in whichone or more fluid directing formations result in a venturi constrictionbeing formed in the fluid flow;

FIG. 4 schematically illustrates another form of the apparatus of FIG.3;

FIG. 5 schematically illustrates an apparatus for extracting power andfor dehumidifying air in an air circuit, according to an embodiment ofthe invention;

FIG. 6 schematically illustrates an embodiment similar to that of FIG.5, except that a cold exhaust from the air turbine is instead used todrive neighbourhood air conditioners or the like; and

FIG. 7 schematically illustrates a desalination plant.

Referring now to FIG. 1, a venturi apparatus is shown in which a fluidflow 110 such as a river flow or sea current is directed past a fluiddirecting formation 160 forming a constriction in the path of the fluidflow. In the example of FIG. 1, the fluid directing formation is in twoparts which smoothly constrict the path of the fluid to a verticaldimension of G. The two parts, one of which is schematically shown asbeing supported from the river or sea bed on supports 165, are shapedrather like an aeroplane wing in cross section, which tends to make thearrangement work well for fluid flow from left to right in the diagram.However, many other shapes, including shapes giving a left-to-rightsymmetrical constriction (i.e. one which works equally well with eitherflow direction) may be used. The fluid velocity is represented as voutside the venturi and u inside.

The fluid directing formation 160 acts to speed up the flow through theconstriction, and so, by the Bernoulli effect, reduces the fluidpressure at the constriction.

An air inlet conduit 145 allows air to flow from the atmosphere to theconstriction in the fluid flow 110. The reduced pressure at theconstriction tends to draw air from the conduit 145 into the fluid flow,in the form of entrained bubbles 155.

For air to be drawn below the fluid surface through the conduit 145, thesuction pressure P must exceed the pressure difference between thesurface of the water and the depth at the bottom of the conduit 145.

The suction pressure P can be expected to be about 5 ρgh, where ρgh isthe pressure due to the head of water generated across the venturiarrangement by its resistance to the flow. The pressure difference dueto depth is ρgH. As the river or sea level rises or falls in the normalcourse of events, both H and h increase, so that the system can workover a reasonable range of fluid levels.

This example illustrates a general principle which will be used invarious embodiments to be described below. However, it is also useful inits own right, as it illustrates a way in which a river can be aerated,for example to improve the health of aquatic life.

FIG. 2 takes this technique one stage further and schematicallyillustrates an apparatus for extracting power from a primary fluid flow.In FIG. 2, a fluid flow 110 (again, a river or a sea current would besignificant examples) entrains a secondary fluid at an entrainmentoutlet 150. A pressure difference is maintained between the points 130and 150, for example by maintaining a siphon arrangement including thenozzle 150. The nozzle 150 may have a diameter of, for example, 1 inch.

The secondary fluid could well be air, as this is a cheap andenvironmentally friendly fluid to entrain into natural waterways, butother entrainment fluids could be used which may or may not be lessdense than the primary fluid. In general, any fluid may be entrainedwith any other, provided the primary fluid is moving and the secondaryfluid is being driven into the flow, or sucked in by some pressuredifference. If a secondary fluid other than air is used, a secondaryfluid source 130 such as a tank or pressurised vessel can be provided.If air is used, and the point of entrainment is at less than atmosphericpressure, the source 130 can be implemented just as a vent to theatmosphere.

As the secondary fluid is entrained in the primary fluid flow, it passesalong a secondary circuit formed of conduits 140 in the direction shownby the arrows. In doing so, its flow can drive a fluid driveable engine120 such as a turbine. In this way, useful work can be extracted fromthe primary fluid flow by the entrainment (and therefore movement) of asecondary fluid in a secondary circuit. Clearly, the fluid driveableengine could drive, for example, an electrical generator, a fan, a pump,a winch or the like (not shown).

FIG. 3 schematically illustrates a similar arrangement in which a fluiddirecting formation 160 (shown in this drawing as a left-rightsymmetrical formation) acts to constrict the primary fluid's flow pathso as to accelerate the primary fluid and reduce the primary fluidpressure at the entrainment outlet. This tends to increase theentrainment of the secondary fluid and therefore its flow rate throughthe secondary circuit. In turn, the increased flow rate allows moreuseful power to be extracted by the fluid driveable engine 120. Forexample, the primary fluid flow may flow through a cross-sectional areaof 1.5 m, narrowing to 0.75 m at the Venturi constriction caused by thefluid directing formations.

FIG. 4 schematically illustrates another example of the apparatus shownin FIG. 3. The conduit 140 is linked to a manifold 170 from which anumber of smaller conduits pass. A number of these smaller conduits forma corresponding number of entrainment outlets 150, 151. This can improvethe efficiency of entrainment by more closely matching the entrainmentoutlet size (e.g<1 mm) to a practical bubble size. A further number ofthe smaller conduits from the manifold 170 can connect to otherVenturi's working in parallel. Multiple manifolds can be used, ormanifolds 151 linked to separate Venturi arrangements all of which canbe linked to a single fluid driveable engine 120.

The conduits feeding the outlets have an area of cross-section which isapproximately the same as the sum of all the outlet areas. The outletsmay be in a ring, in the case of a cylindrical Venturi, about 1 inchapart.

If the purpose of admitting air to the flow is aeration, then air isbest admitted via a large number of small orifices, or even through aporous material such as a porous stone. This keeps the bubble size low.If the purpose is to generate air (pneumatic) power, the bubbles can bebigger, but small bubbles are generally better than big bubbles becausethe speed at which they rise is less. In all cases, the amount ofsecondary fluid entrained will increase with the number and size of theinlet conduits, until the point is reached when the primary flow isbeing disrupted.

FIG. 5 schematically illustrates an apparatus for extracting power andfor dehumidifying air in an air circuit.

In FIG. 5, a primary fluid (in this example, the sea) flows in a flowdirection 210. The primary fluid also flows through a primary fluidcircuit formed of an inlet 250, conduits 255, a tank 270 and outlets275. The outlets 275 are disposed in a venturi arrangement 410 formed offluid directing formations (not shown) constricting the flow path of theprimary fluid so as to increase its velocity and decrease its pressure.So, the flow of the primary fluid through the primary fluid circuit isdriven by the pressure difference between the (higher) pressure at theinlet 250 and the (lower) pressure at the outlets 275. It may also beassisted by the fact that the inlet 250 preferably faces into theoncoming flow of the primary fluid.

A secondary fluid (e.g. air) flows around a secondary fluid circuitcomprising conduits 265, a heat exchanger 290, a fluid driveable engine(e.g. a turbine) 230, and a nozzle 380. The nozzle 380 is disposed inthe flow path of the primary fluid in the primary fluid circuit. So, theflow of the secondary fluid is as follows. The secondary fluid is drawninto and entrained in the flow of the primary fluid at the nozzle 380,by the negative hydrostatic pressure ρ₀gh_(c) at that point inside thesiphon arrangement. The secondary fluid is drawn down to the tank 270 inthe form of entrained bubbles. In the tank 270, there is the opportunityfor these entrained bubbles to come out of the primary fluid. Thesecondary fluid released in this way passes up to the heat exchanger 290and from there to the fluid driveable engine 230. At the exhaust (lefthand) end of the fluid driveable engine 230, the secondary fluid passes,via a valve 350, back to the nozzle 380.

So, in basic terms the system operates in a similar manner to thosedescribed above, in that the fluid driveable engine 230 is driven by theflow of the secondary fluid to extract useful power from the main flow210 of the primary fluid. The secondary fluid is itself driven by theflow of the primary fluid through a primary fluid circuit, effectivelyby a siphon arrangement.

Vent valves 310, 330 and 390 allow excess air or water to be vented tothe atmosphere or allow extra air to be introduced at various points inthe system.

The entrained bubbles can be carried downwards by the primary fluid flowas long as that flow is more than about 0.2 m/s. This is because thespeed at which bubbles rise in still water is of the order of 0.2 m/s.In fact, a downward primary fluid velocity of several m/s could beexpected. The speed of this downward flow can be expected to reduce nearto the walls of the conduit, so bubbles which stray far from the axismay no longer be carried downwards. To avoid this problem, bubbles couldbe retained nearer to the axis of the conduit by using internal vanes orsimilar fluid directing formations to make the downward flowing fluidspin slightly about a longitudinal (vertical) axis.

For this arrangement to work, the suction provided by the venturiarrangement has to overcome the differential hydraulic pressureg(h₁+h₂)(ρ₁−ρ₂) where ρ₁ is the density of water and ρ₂ is the averagedensity of a column of water with entrained bubbles. By adjusting therate at which the secondary fluid flows in, a good match can be obtainedbetween the differential hydraulic pressure and the suction from theventuri arrangement, providing efficient power transfer.

In an ideal (theoretical no-losses) situation, the head of water ΔH(approx 5 h) across the venturi arrangement is entirely due to the loadimposed on it by drawing down water loaded with bubbles.

However, there is also another beneficial effect of this apparatus. Thisis that the air passing up from the tank 270, which will be damp airhaving been entrained in the water flow, is at least partiallyfreeze-dried by the cool air generated by expansion through thefluid-driveable engine (e.g. a turbine such as a rotary vane turbine, areciprocating engine such as a piston in a cylinder, or a liquid ringexpander). The purpose of drying the air is to increase the life of thefluid-driveable engine by removing the water at its input, which watermay be salty in the case of a marine installation. Considering theexample of a turbine, as the air temperature goes down when the airexpands through the turbine, any water vapour in the air would freezebringing the salt out of solution. Unless action (such as pre-drying theair) is taken, the high speed turbine blades would be bombarded by icecrystals and salt crystals, which could lead to rapid erosion.

FIG. 6 schematically illustrates a very similar system, except that aheat exchanger 290′ is connected to a heat exchange circuit (e.g. achilled water circuit) of a nearby plant such as an air conditioningplant. So, the expansion through the fluid driveable engine 230 providescooling which can be utilised in the nearby plant. This benefit is inaddition to the power generated from the fluid driveable engine 230.

FIG. 7 schematically illustrates a desalination plant using some ofthese principles. The main differences between FIG. 7 and FIG. 5 arethat a compressor 420 and a fan 430 are driven by the fluid driveableengine 230 (ideally, instead of any other load such as a generator). Thecompressor 420 tends to compress the air to be entrained into a downwardprimary fluid flow, increasing the entrainment efficiency. The fan 430draws damp air from a warm sea breeze or a solar pond over a beatexchanger 290″ carrying cool air from the exhaust of the fluid driveableengine. As the warm damp air passes over the heat exchanger 290″, atleast a part of the moisture it carries condenses to form fresh waterwhich emerges at an outlet 440.

1. An apparatus for extracting power from a fluid flow, the apparatuscomprising: a fluid driveable engine, a conduit, said conduit disposedto enable fluid communication between a portion of the fluid flow, thefluid driveable engine and a transmission fluid, the transmission fluidbeing a gas and the fluid flow being a liquid, a portion of the fluidflow being at a lower pressure than the pressure of the transmissionfluid by virtue of the fluid flow rate, thus causing the transmissionfluid to be drawn through the conduit, said transmission fluid exitingthe conduit via a plurality of entrainment outlets to become entrainedin the fluid flow, the fluid driveable engine being arranged such thatthe flow of the transmission fluid along the conduit acts to drive thefluid driveable engine.
 2. Apparatus as claimed in claim 1, comprising:at least one fluid directing formation formed to define a channel in thefluid flow having a flow accelerating constriction shaped such that thefluid in the channel, is caused to accelerate as it flows through theflow accelerating constriction of the channel.
 3. Apparatus according toclaim 1, in which the fluid flow comprises a flow along a conduitbetween two positions in a fluid stream, a conduit inlet position beingat a higher fluid pressure than a conduit outlet position by virtue of alower pressure velocity at the conduit outlet position.
 4. Apparatusaccording to claim 3, comprising a fluid directing formation forconstricting the fluid stream at the conduit outlet position withrespect to the fluid stream at the conduit inlet position.
 5. Apparatusas claimed in claim 1, wherein the fluid flow comprises a flow of water.6. Apparatus as claimed in claim 1, wherein the transmission fluidcomprises air.
 7. Apparatus according to claim 1, in which the fluiddriveable engine comprises a turbine.
 8. Apparatus according to claim 7,comprising a heat exchanger in the transmission fluid flow path at atransmission fluid exhaust of the turbine.
 9. Apparatus according toclaim 8, in which the heat exchanger is arranged to cool thetransmission fluid.
 10. Apparatus according to claim 8, in which theheat exchanger is arranged to cool a further transmission fluid incommunication with external plant.
 11. Apparatus according to claim 8,in which the heat exchanger is arranged to condense water vapour fromambient air.
 12. Apparatus according to claim 1, wherein the conduit islinked to manifold from which a plurality of smaller conduits pass, eachof said smaller conduits comprising an entrainment outlet.
 13. Apparatusaccording to claim 1, wherein said plurality of entrainment outlets areformed within a porous material.
 14. Apparatus according to claim 1, theconduit comprising fluid directing formation, the fluid directionformations being arranged so as to cause downward flowing fluid to spinabout a longitudinal axis.